Channel electron multipliers 10 (CEMs) (FIG. 1) and microchannel plates 20 (MCPs) (FIG. 2) are efficient, low-noise, vacuum-electron amplifiers with typical gains (G)=I.sub.o /I.sub.i in the range of 10.sup.3 -10.sup.8 where I.sub.o /I.sub.i is the ratio of the output to input currents. CEMs 10 are devices which have a single channel 12 and are generally used for direct detection of charged particles (e.g., electrons and ions) and photons from soft X-ray to extreme ultraviolet wavelengths (i.e., 1-100 nm). They are mainly used as detectors in a wide variety of scientific instrumentation for mass spectrometry, electron spectroscopy for surface analysis, electron microscopy, and vacuum ultraviolet and X-ray spectroscopy.
MCPs 20 are fabricated as areal arrays of millions of essentially independent channel electron multipliers which operate simultaneously and in parallel. Using an MCP, direct detection of charged particles and sufficiently energetic electromagnetic radiation can be achieved in two dimensions over large areas (up to several hundred cm.sup.2) with good resolution (channel spacing or pitch&lt;0 .mu.m) at fast response times (output pulse widths&lt;300 ps), and with linear response over a broad range of input even levels (10.sup.-12 -10.sup.-1 A). By placing an MCP between a suitable photocathode and fluorescent screen in an optical image tube (not shown), two-dimensional signals from the ultraviolet to the near-infrared spectral region can be intensified and displayed as a visible image. While MCPs continue to find major application in image tubes for military night-vision systems, there is now growing interest in MCPs for high-performance commercial applications as well. These presently include high-speed and high-resolution cameras, high-brightness displays, and state-of-the-art detectors for scientific instrumentation.
CEMs and MCPs essentially consist of hollow, usually cylindrical channels. When operated at pressures .ltoreq.1.3.times.10.sup.-4 Pa (10.sup.-6 torr) and biased by an external power supply, such channels support the generation of large electron avalanches in response to a suitable input signal. The cutaway view of FIG. 1 shows CEM 10 in operation. The process of electron multiplication in a straight channel does not critically depend on either the absolute diameter (D) or length (L) of the channel, but rather on the L/D ratio (.alpha.). For a curved channel, the ratio (.beta.) of the channel length L to the radius of channel curvature (S), L/S, is the important parameter. These geometric ratios largely determine the number of multiplication events (n) that contribute to the electron avalanche. Typical values of .alpha. range from 30 to 80 for conventional CEMs and MCPs with channel diameters D on the scale of 1 mm and 10 .mu.m, respectively. Thus, a CEM 10 is a single channel electron multiplier of macroscopic dimensions while MCP 20 is a wafer-thin array of microscopic electron multipliers with channel densities of 10.sup.5 -10.sup.7 /cm.sup.2.
The channel wall 14 of CEM 10 or the wall 24 of the MCP 20 acts as a continuous dynode for electron multiplication and may be contrasted with the operation of photoemissive detectors using discrete dynodes (e.g., an ordinary photomultiplier tube). In operation, the continuous dynodes 14 and 24 must be sufficiently resistive to support a biasing electric field (.epsilon.)=10.sup.2 -10.sup.5 V/cm without drawing an excessive current. They must also be conductive enough such that a discharging current is available to replenish electrons emitted from the dynode 14,24 during an electron avalanche. For example, when a signal event 30 such as an electrically charged particle (FIG. 1) (e.g., an electron or a Ne.sup.+ ion) or sufficiently energetic radiation (e.g., an X-ray photon) strikes the channel wall 14 near the negatively biased input end 32, there is a good probability that electrons 34 will be ejected from the surface 14. These primary electrons 34 are accelerated down the channel 12 by an applied electric field .epsilon. (see arrow 36) produced by the bias potential (V.sub.B) represented by the power supply 38. .epsilon.=V.sub.B /L, where V.sub.B in volts.perspectiveto. 20-25.alpha. for a conventional straight-channel multiplier. Collision of the emitted electrons 34 with the channel wall 14 causes the emission of secondary electrons 40. These secondary electrons in turn act as primary electrons in subsequent collisions with the channel wall 14 which produce another generation of secondary electrons. Provided that more than one secondary electron is emitted for every incident primary electron, the secondary electron yield (.delta.) &gt;1, and n repetitions of this primary collision-secondary emission sequence in the direction of the output end 41 rapidly leads to an output electron avalanche 42 of magnitude .delta..sup.n.
The near-surface region of the dynode 14 must have an average value of .delta. sufficiently greater than unity to support efficient multiplication of primary electrons impinging on a channel wall with energies (E.sub.P) mostly in the range of 20-100 eV. For materials with good secondary electron emission properties, .delta.initially increases with E.sub.P from .delta.&lt;1 to .delta.=1 at the first crossover energy E.sub.P.sup.I, and then to .delta.&gt;1. Emissive materials of greatest interest for electron multipliers tend to have values of E.sub.P.sup.I in the range of about 10 eV.ltoreq.E.sub.P.sup.I .ltoreq.50 eV, the smaller the better. For such materials, a linear approximation of .delta.(E.sub.P) is .delta.=E.sub.P /E.sub.p.sup.I for E.sub.P .ltoreq.100 eV. As an example, if E.sub.P.sup.I =30 eV for the continuous dynodes in conventional CEMs and MCPs, then an estimate of the range of .delta. for primary electrons with E.sub.P =20-100 eV is 0.7.ltoreq..delta..ltoreq. 3.3. Now, for a straight channel multiplier with .alpha.=40, V.sub.B =1000 V, E.sub.P.sup.I =30 eV, and a most probable initial energy E.sub.S =3 eV for a secondary electron as it emerges from the dynode surface, the electron gain G from a single input electron approximately calculated as follows: ##EQU1## The most probable collision energy of the primary electrons EQU (E.sub.P)=(qV.sub.B).sup.2 /4E.sub.S .alpha..sup.2 .perspectiveto.52 eV;
the average yield or gain per multiplication event EQU .delta.=(qV.sub.B).sup.2 /4E.sub.S E.sub.P.sup.I .alpha..sup.2 .perspectiveto.1.75;
the number of the multiplication events EQU n=4E.sub.S .alpha..sup.2 /qV.sub.B .perspectiveto.19; and,
q is the magnitude of electronic charge.
When the electron avalanche emerges from the channel as an output signal, it typically represents a very large amplification of the original input signal. Because electron multiplication increases geometrically down the length of a channel, signal gains G ranging from 10.sup.3 to 10.sup.8 can be obtained depending upon the specific dynode materials, channel geometry, detector configuration, and application.
Straight-channel multipliers are limited to electron gains of about 104 due to a phenomenon known as positive ion feedback. Near the output end of a channel multiplier and above some threshold gain, residual gas molecules within the channel or gasses adsorbed on the channel wall can become ionized by interaction with the electron avalanche. In contrast to the direction of travel for electrons with negative electrical charge, positive ions are accelerated toward the negatively-biased input end of the channel. Upon striking the channel wall, these ions cause the emission of electrons which are then multiplied geometrically by the process described above. Spurious and at times regenerative output pulses associated with ion feedback can thus severely degrade the signal-to-noise characteristics of the detector.
An effective method for reducing ion feedback channel multipliers is to curve the channel. Channel curvature restricts the distance that a positive ion can migrate toward the input end of a channel, and hence greatly reduces the amplitude of spurious output pulses. Single MCPs with straight channels typically provide electron gains of 10.sup.3 -10.sup.4. Curved-channel MCPs can produce gains of 10.sup.5 -10.sup.6 but are difficult and expensive to manufacture. Curved-channel CEMs can operate at gains in excess of 10.sup.8.
MCPs 20 are usually fabricated with channels 22 that are inclined at an angle of .about.10.degree. relative to a normal projection from the flat parallel surfaces 26 of the device. This is done to improve the first strike efficiency of an input event. Stacking MCPs and alternating the rotational phase of the channel orientation by 180.degree. provides another means for overcoming ion feedback in MCP detectors. Two-stage (Chevron.TM.) and three-stage (Z-stack) assemblies of MCPs thereby produce gains of 10.sup.6 -10.sup.7 and 10.sup.7 -10.sup.8, respectively.
The channel wall of a CEM or MCP acts as a continuous dynode for electron multiplication and may be contrasted elsewhere with the operation of detectors using discrete dynodes (e.g., an ordinary photomultiplier tube). A continuous dynode must be sufficiently conductive to replenish electrons which are emitted from its surface during an electron avalanche. In analog operation of CEMs and MCPs at a given gain G, the output current I.sub.o from a channel is linearly related to the input current providing the output does not exceed about 10% of the bias current (i.sub.B), imposed by V.sub.B, in the channel wall. Above a threshold input level, I.sub.i .about.0.1 i.sub.B /G, gain saturation occurs and current transfer characteristics are no longer linear. On the other hand, the continuous dynode must also be resistive enough to support a biasing field .epsilon.=10.sup.2 -10.sup.5 V/cm without drawing an excessive i.sub.B, as manifest by thermal instability that is associated with Joule heating. Moreover, the near-surface region of the dynode must have an average value of .delta.sufficiently greater than unity to support efficient multiplication of electrons impinging on a channel wall, as discussed above.
The electrical and electron emissive properties of continuous dynodes in the current generation of CEMs MCPs critically depend on details of their manufacture. MCPs are presently fabricated by a glass multifiber draw (GMD) process that includes drawing a rod-in-tube glass fiber of a barium borosilicate core glass clad with a lead silicate glass; stacking the composite fiber into hexagonal array and redrawing glass multifiber bundles; stacking of multifiber bundles and consolidating into billet consisting of an array of solid core glass channels imbedded in a cladding glass matrix; wafering of the billet and surface finishing; wet chemical processing to remove the core glass leaving behind an array of hollow channels extending through a wafer of cladding glass; additional wet chemical processing to enhance secondary emission from the channel surface; reducing the lead silicate glass in hydrogen atmosphere to render the dynode surface electronically conductive with a sheet resistance (R.sub.S)=10.sup.11 -10.sup.14 .OMEGA./sq; and electroding of the flat surfaces of the MCP wafer.
Fabrication of CEMs is simpler; it entails thermal working of lead silicate glass tubing into a suitable geometry; reducing the glass in hydrogen to produce a continuous dynode surface with R.sub.S =10.sup.6 -10.sup.8 .OMEGA./sq, and electroding. On account of the vastly different values of R.sub.S that are required for continuous dynodes in MCPs versus CEMs, compositionally distinct lead silicate glasses have been formulated for each application.
The hydrogen reduction step is essential to the operation of conventional electron multipliers. Lead cations in the near-surface region of the continuous glass dynode are chemically reduced in a hydrogen atmosphere at temperatures of about 350.degree.-500.degree. C. from the Pb.sup.2 state to lower oxidation states with the evolution of H.sub.2 O as a reaction product. The development of significant electronic conductivity in a region no more than about 1 .mu.m beneath the surface of reduced lead silicate glass (RLSG) dynodes has been explained in two rather different ways. One theory holds that a small fraction (i.e., .about.10.sup.6) of the lead atoms within the reaction zone remains atomically dispersed in multiple valence states (i.e., Pb.sup.1+ and Pb.sup.2+). An electron hopping mechanism via localized electronic states in the band gap, associated with lead atoms in the multiple valence states, is said to give rise to electronic conduction. Another theory, noting that most of the lead atoms within the reaction zone are reduced to the metallic state and are agglomerated into droplet-like particles with a discontinuous morphology, suggests that electronic conduction derives from a tunneling mechanism between such particles. Regardless of the mechanism that ultimately proves correct, one can expect that the electrical characteristics of RLSG dynodes are a complex function of the chemical and thermal history of the glass surface as determined by the details of its manufacture.
During hydrogen reduction, other high-temperature processes including diffusion and evaporation of mobile chemical species in the lead silicate glass (e.g., alkali alkaline earth, and lead atoms) also act to modify the chemistry and structure of RLSG dynodes. Compositional profiles through the near-surface region of glasses that are used in the manufacture of MCPs have indicated that RLSG dynodes have a two-layer structure.
An exemplary RLSG dynode 50, shown in FIG. 3, comprises a superficial silica-rich and alkali-rich, but lead-poor dielectric emissive layer 52 about 2-20 nm in thickness (d) that produces adequate secondary emission (i.e., E.sub.P.sup.I .about.30 eV) to achieve useful electron multiplication. Beneath this dielectric emissive layer 52 (or dynode surface), a semiconductive lead-rich layer 54 about 100-1000 nm in thickness (t) serves as an electronically conductive path for discharging the emissive layer 52. Upon consideration of the ranges of R.sub.S for RLSG dynodes given above and assuming the semiconductive layer 54 has a thickness t=100 nm, it can be readily shown that, the bulk electrical resistivity (r) of the material comprising semiconductive layer 54 is r=R.sub.S.t=10.sup.1 -10.sup.3 .OMEGA..cm for CEM dynodes and r=10.sup.6 -10.sup.9 .OMEGA..cm for MCP advances A base glass 56 provides mechanical support for the continuous RLSG dynode 50 in the geometry of macroscopic channels for CEMs or arrays of microscopic channels for MCPs. The interface 58 shown schematically in FIG. 3 between the conductive 54 and emissive 52 layers in actuate RLSG dynodes is rather less distinct than illustrated in FIG. 3; this schematic structure, however, does provide a useful model.
While the manufacturing technology of RLSG MCPs and CEMs is mature, relatively inexpensive, and reasonably efficient, it imposes important 1 imitations on cur rent device technology and its future development. These limitations are summarized as follows. Both electrical and electron emissive properties of RLSG dynodes are quite sensitive to the chemical and thermal history of the glass surface comprising the dynode. Therefore, reproducible performance characteristics for RLSG MCPs and CEMs critically depend upon stringent control over complex, time-consuming, and labor-intensive manufacturing operations. In addition, the ability to enhance or tailor the characteristics of RLSG MCPs and CEMs is constrained by the limited choices of materials which are compatible with the present manufacturing technology. Gain stability, maximum operating temperature, background noise, and heat dissipation in high-current devices are several key areas where performance is adversely affected by material limitations of the lead silicate glasses that are used in the manufacture of conventional MCPs and CEMs.
The GMD process also imposes important manufacturing constraints on the geometry, and hence on the performance and applications of RLSG MCPs in the following ways:
channel diameters .gtoreq.4 .mu.m and channel pitches .gtoreq.6 .mu.m in current practice limit temporal and spatial resolution; PA1 quasi-periodic arrays of channels within multifiber regions and gross discontinuities at adjacent multifiber boundaries greatly complicate the task of addressing or reading out individual or small blocks of channels; PA1 variations in channel diameter from area to area in an array are manifest as patterns with differential gain; and PA1 the largest size of a microchannel array is now limited to a linear dimension on the order of 10 cm.
U.S. Pat. No. 5,086,248, filed Aug. 18, 1989 concurrently with this application of Horton et al., U.S. Pat. No. 5,086,248 which issued Feb. 4, 1992, Ser. No. 395,596, filed Aug. 18, 1989, which is incorporated herein by reference address these problems. The disclosure of Horton et al. is incorporated herein by reference. The Horton et al. patent discloses how to achieve reduced channel size and pitch less than 10 .mu.m and particularly shows how to achieve channel diameters less than 4 .mu.m and channel-to-channel spacing of less than 6 .mu.m. The present invention allows these small diameter and close pitch devices to be effectively utilized.
Finally, despite the major market for MCPs in military night vision devices, other substantial applications for these remarkable detectors have been slow to evolve in part because they are difficult to interface with solid-state electronics. Greater compatibility with semiconductor electronics (e.g., with regard to materials of construction, interconnection, or power requirements for operation) would facilitate the implementation of important new applications including commercial night vision, optical computing, and high-performance display, photographic, and imaging technologies.